Optical waveguide termination with vertical and horizontal mode shaping

ABSTRACT

An optical device is disclosed which includes a waveguide that support a first optical mode in a first region and a second optical mode in a second region. The waveguide further includes single material guiding layer having a lower portion with a first taper and an upper portion with a second taper.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication Ser. No. 60/255,868 filed Dec. 14, 2000, entitled “OpticalWaveguide Termination With Vertical and Horizontal Mode Shaping.”, andU.S. Provisional Patent Application Ser. No. 60/287,032 filed Apr. 30,2001, entitled “Optical Waveguide Termination With Vertical andHorizontal Mode Shaping.” The disclosure of the above referencedprovisional patent applications is specifically incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates generally to optical integrated circuits(OIC), and particularly to a structure for coupling optical waveguides.

BACKGROUND OF THE INVENTION

Optical communications are evolving as the chosen technique for data andvoice communications. OIC's are often used at the point of transmission,reception and amplification. Optical fibers may be coupled to the OIC toenable the optical connection of the OIC other components of an opticalcommunications system. Typically, planar waveguides are used toefficiently couple light to and from active and passive devices of theOIC. The planar waveguides are often made of relatively high refractiveindex materials to facilitate desired integration and miniaturization ofthe OIC. Coupling between the OIC and the optical communication systemis often achieved by coupling optical fibers of the system to planarwaveguides of the OIC.

While clearly beneficial to the integration and miniaturization of OICs,the planar waveguides commonly used in these circuits do not efficientlycouple directly to optical fibers. To this end, planar opticalwaveguides and optical fiber waveguides used in high-speed and long-hauloptical transmission systems often are designed to support a singlemode. Stated differently, the waveguides are designed such that the waveequation has one discrete solution; although an infinite number ofcontinuous solutions (propagation constants) may be had. The discretesolution is that of a confined mode, while the continuous solutions arethose of radiation modes.

Because each waveguide will have a different discrete (eigenvalue)solution for its confined mode, it is fair to say that two disparatewaveguides, such as an optical fiber and a planar waveguide, generallywill not have the same solution for a single confined mode. As such, inorder to improve the efficiency of the optical coupling, it is necessaryto have a waveguide transition region between the planar waveguide ofthe OIC and the optical fiber. This transition region ideally enablesadiabatic compression or expansion of the mode so that efficientcoupling of the mode from one type of waveguide to another can becarried out.

As mentioned, optical fibers typically support mode sizes(electromagnetic field spatial distributions) that are much larger, bothin the horizontal and vertical directions than modes supported by higherindex waveguide structures, such as planar waveguides. Therefore, achallenge is to provide a waveguide transition region that enablesadiabatic expansion of the mode so that it is supported by to theoptical fiber. Moreover, it is useful to achieve the adiabatic expansionof the mode in both the horizontal and vertical directions.

Fabricating a waveguide to effect adiabatic expansion of the mode in thevertical direction has proven difficult using conventional fabricationtechniques. To this end, tapering the thickness of the waveguide toaffect the vertical adiabatic expansion of the mode is exceedinglydifficult by conventional techniques.

What is needed therefore is a structure for effecting efficient couplingbetween waveguides having disparate characteristic mode sizes whichovercomes the drawbacks of the prior art described above.

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention an opticaldevice is disclosed which includes a waveguide that supports a firstoptical mode in a first region and a second optical mode in a secondregion. The waveguide further includes a single material guiding layerhaving a lower portion with a first taper and an upper portion with asecond taper.

According to another exemplary embodiment of the present invention, anoptical device is disclosed which includes a waveguide having a singlematerial guiding layer. The single material guiding layer has a lowerportion, which tapers from a first width to a second width, and an upperportion which tapers from the first width to a point.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, for purposes of explanation andnot limitation, exemplary embodiments disclosing specific details areset forth in order to provide a thorough understanding of the presentinvention. However, it will be apparent to one having ordinary skill inthe art having had the benefit of the present disclosure, that thepresent invention may be practiced in other embodiments that depart fromthe specific details disclosed herein. Moreover, descriptions ofwell-known devices, methods and materials may be omitted so as to notobscure the description of the present invention.

FIG. 1(a) is a top view of a waveguide according to an illustrativeembodiment of the present invention.

FIG. 1(b) is a perspective view of the waveguide shown in FIG. 1(a).

FIG. 1(c) is a side elevational view of FIG. 1(a) of a waveguideaccording to an illustrative embodiment of the present invention.

FIG. 2(a) is a perspective view of a waveguide coupled to an opticalfiber in accordance with an illustrative embodiment of the presentinvention.

FIG. 2(b) is a top view of a waveguide according to an illustrativeembodiment of the present invention.

FIGS. 3(a)-3(f) are graphical representations of the electric fielddistributions of optical modes at various regions of a waveguideaccording to an illustrative embodiment of the present invention.

FIGS. 4(a)-4(d) are top views of guiding layers of waveguides inaccordance with illustrative embodiments of the present invention.

FIG. 5 is a perspective view of an illustrative embodiment of thepresent invention.

FIG. 6 is a perspective view of an illustrative embodiment of thepresent invention.

DEFINED TERMS

1. As used herein, the term “on” may mean directly on or having one ormore layers therebetween.

2. As used herein, the term “single material” includes materials havinga substantially uniform stoichiometry. These materials may or may not bedoped. Illustrative materials include, but are in no way limited tosilicon, SiO_(x)N_(y), SiO_(x), Si₃N₄, and InP. Moreover, as usedherein, the term single material includes nanocomposite materials,organic glass materials.

3. As used herein, the term “bisect” may mean to divide into two equalparts. Alternatively, the term “bisect” may mean to divide into twounequal parts.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, exemplary embodiments disclosing specific details areset forth in order to provide a thorough understanding of the presentinvention. However, it will be apparent to one having ordinary skill inthe art having had the benefit of the present disclosure, that thepresent invention may be practiced in other embodiments that depart fromthe specific details disclosed herein. Moreover, descriptions ofwell-known devices, methods and materials may be omitted so as to notobscure the description of the present invention.

Briefly, the present invention relates to an optical waveguide whichfosters adiabatic mode expansion/compression thereby enabling opticalcoupling between a first waveguide, which supports a first optical modeand a second waveguide, which supports a second optical mode. Accordingto an exemplary embodiment, the waveguide supports a first optical modein a first region and a second optical mode in a second region. Thewaveguide includes a single material guiding layer having a lowerportion with a first taper and an upper portion with a second taper.According to another exemplary embodiment of the present invention, anoptical device is disclosed which includes a waveguide having a singlematerial guiding layer. The single material guiding layer has a lowerportion, which tapers from a first width to a second width, and an upperportion which tapers from the first width to a point. The singlematerial may be disposed on a stress compensating layer, which is usedto reduce stress induced polarization mode dispersion and temperatureinduced polarization mode dispersion. This stress compensating layerwill not substantially impact the optical characteristics of awaveguide.

The waveguide according to exemplary embodiments described herein may bean integral part of an OIC, formed during the fabrication of the OIC.The waveguide illustratively couples a planar waveguide of the OIC to anoptical fiber of an optical communications system. Of course, multiplewaveguides may be used to couple multiple optical fibers at variouslocations of the OIC.

FIGS. 1(a) and 1(b) show a waveguide 100 according to an illustrativeembodiment of the present invention. A guiding layer 101 is disposed ona lower cladding, layer 102. The guiding layer 101 is illustratively asingle material. An upper cladding layer (not shown) covers the guidinglayer 101. The indices of refraction of the upper and lower claddinglayers may or may not be the same. In all cases, the indices ofrefraction of the upper and lower cladding layers are less than theindex of refraction (n_(g)) of the guiding layer 101. The waveguide 100includes a first region 103 and a second region 104. The guiding layer101 further includes an upper portion 105 and a lower portion 106. Theupper portion 105 tapers at an angle θ₂ relative to the edge 107 of theguiding layer 101. The lower portion 106 tapers at an angle θ₁ relativeto the edge 107 of the guiding layer 101.

Reducing the thickness and width of the guiding layer 101 effectssubstantially adiabatic expansion/compression of an optical modetraversing the waveguide. (As would be readily apparent to one havingordinary skill in the art, adiabatic expansion of a mode occurs when themode is traveling in the +z-direction; while from the reciprocityprinciple of optics, adiabatic compression occurs when the mode istraveling in the −z-direction). As the width of the guiding layer 101reduces along a first taper 108 from a width w, to effectively zerowidth at termination point 109, the effective index of refraction isreduced. Moreover, the guiding layer 101 reduces along second taper 111from a width w₁ to a width w₂, a finite width, at endface 110. Again,the effective index of refraction decreases as the width of the guidinglayer 101 decreases. Due to the reduction in the effective index ofrefraction, the horizontal portion of the optical mode expands (is lessconfined in the guiding layer 101) as the mode traverses the waveguidein +z-direction. Fabrication of the first taper 108 and second taper 111of the guiding layer 101 may be carried out by well known techniques, asdescribed in further detail below.

Of course, it is also useful to adiabatically expand/compress thevertical portion of the optical mode. In order that the vertical portionof the optical mode undergoes substantially adiabaticexpansion/compression, the thickness of the guiding layer is reduced.

Turning to FIG. 1(c), a side-elevational view of an illustrativeembodiment of FIG. 1(a) is shown. In this embodiment, the thickness ofguiding layer 101 reduces in the +z-direction from a thickness t₁ to athickness t₂ as shown. An upper cladding layer (not shown) may cover theguiding layer 101. While the single material used for guiding layer 101has an index of refraction n_(g), as the thickness of the guiding layer101 is reduced from a thickness t₁ to a thickness t₂, the effectiveindex of refraction is reduced. Accordingly, the vertical portion of anoptical mode traversing the guiding layer 101 in the +z-direction willexpand, as it is less confined to the guiding layer 101. Finally,according to the illustrative embodiment of the present invention shownin FIGS. 1(a) and (b), the endface 110 of the guiding layer 101 has awidth w₂, thickness t₂ and index of refraction that produce an opticalmode well matched to that of an optical fiber. Accordingly, the singleoptical mode supported by the waveguide 100 at endface 110 will also beone which is supported by an optical fiber. As such, good opticalcoupling between the guiding layer 101 of the waveguide 100 and theguiding layer of an optical fiber (not shown) results.

The waveguide 100 according to exemplary embodiments of the presentinvention may be fabricated so that the upper portion and lower portionof the guiding layer 101 are symmetric about a plane whichlongitudinally bisects the guiding layer 101. Alternatively, thewaveguide 100 according to exemplary embodiments of the presentinvention may be fabricated so that the upper portion, or the upperportion and the lower portion, of the guiding layer 101 are asymmetricabout an axis which bisects the waveguide 100. These and other exemplaryembodiments of the present invention are described in the examplesdescribed below.

EXAMPLE I

Turning to FIG. 2(a), a perspective view of a waveguide 200 according toan exemplary embodiment of the present invention is shown. A lowercladding layer 202 is disposed on a substrate 201. A guiding layer 203is disposed on lower cladding layer 202. Waveguide 200 has a firstregion 204 and a second region 205. The guiding layer 203 includes alower portion 206 and an upper portion 207. An optical mode is coupledfrom an endface 209 to an optical fiber 208. For the purposes of ease ofdiscussion, an upper cladding layer is not shown in FIG. 2(a). Thisupper cladding layer would cover the guiding layer 203. The uppercladding layer, guiding layer 203 and lower cladding layer 202 form awaveguide 200 according to an illustrative embodiment of the presentinvention. The upper cladding layer may have the same index ofrefraction as the lower cladding layer 202. Alternatively, the uppercladding layer may have a higher (or lower) index of refraction than thelower cladding layer 202. The guiding layer 203 has an index ofrefraction, n_(g), which is greater than the indices of refraction ofboth the upper cladding layer and lower cladding layer 202. Finally,according to the illustrative embodiment of the present example of theinvention, the upper portion 207 and lower portion 206 are symmetricabout an axis 213 that bisects guiding layer 200 (shown below).

As mentioned above, it may be desirable to couple the optical fiber 208to an OIC (not shown). This coupling may be achieved by coupling theoptical fiber to a planar waveguide (not shown) of the OIC. However, theplanar waveguide supports a first optical mode and the optical fiber 208supports a second optical mode. As such, the first optical mode of theplanar waveguide will not be supported by the optical fiber in anefficient manner, and a significant portion of the energy of the firstoptical mode of the planar waveguide could be transformed into radiationmodes in the optical fiber 208.

Waveguide 200 may be disposed between the planar waveguide of the OICand the optical fiber 208 to facilitate efficient optical couplingtherebetween. To this end, as described in detail above, the firstoptical mode of the planar waveguide is physically more confined to theguiding layer of the planar waveguide than the second optical mode is inthe guiding layer of the optical fiber. That is, the confined mode ofthe planar optical waveguide is smaller than the confined mode of anoptical fiber. Accordingly, waveguide 200 is useful in efficientlytransferring the energy of the first optical mode of the planarwaveguide into optical fiber 208 by a substantially adiabatic expansionof the mode. Stated differently, the solution to the wave equation forthe planar waveguide is a first optical mode. As the supported mode ofthe planar waveguide traverses the waveguide 200 it undergoes atransformation to a second optical mode that is supported by acylindrical optical waveguide (optical fiber 208).

Advantageously, the transformation of the mode which is supported by theplanar waveguide, to a mode which is supported by waveguide 200, andultimately to a mode which is supported by optical fiber 208, issubstantially an adiabatic transformation. As such, transition lossesfrom the planar waveguide to the optical fiber 208 are minimal.Illustratively, transition losses are approximately 0.1% or less.Moreover, the second region 205 of the waveguide 200 effects bothhorizontal and vertical transformation of the mode. Finally, the abovediscussion is drawn to the adiabatic expansion of a mode in waveguide200. Of course, from the principle of reciprocity in optics, a modetraveling from optical fiber 208 (−z-direction) to a planar waveguidewould undergo an adiabatic compression by identical principles ofphysics.

FIG. 2(b) shows a top-view of the waveguide 200 of FIG. 2(a). Theguiding layer 203 of waveguide 200 includes a first region 204 which iscoupled to (or is a part of) another waveguide, such as a planarwaveguide (not shown). The second region 205 is the region in which thetransformation of the mode supported in the planar waveguide into onewhich is supported by another waveguide (e.g. optical fiber 208) occurs.This second region 205 includes a lower portion 206 and an upper portion207. Upon reaching the end face 209, the single confined mode is onewhich is supported by optical fiber 208. Accordingly, a significantproportion of the energy of the mode is not lost to radiation modes inthe optical fiber. In summary, the structure of the illustrativeembodiment of FIG. 2(a) and FIG. 2(b) results in efficient coupling ofboth the horizontal portion and the vertical portion of the opticalmode. The structure is readily manufacturable by standard semiconductorfabrication techniques.

As shown is FIG. 2(b), as the guiding layer 203 tapers, the lowerportion 206 is at a first angle, θ₁, relative to the edge of waveguide203; and the upper portion 207 is at a second angle, θ₂, again relativeto the edge of waveguide 203. Illustratively, the angles are in therange of approximately 0° to approximately 0.5°. Sometimes, it ispreferable that the angles are in the range of greater than 0° toapproximately 0.5°. As can be readily appreciated by one having ordinaryskill in the art, the greater the angle of the taper, the shorter thelength of the taper. Contrastingly, the smaller the taper angle, thelonger the length of the taper. As will be described in greater detailherein, a greater taper length may require more chip area, which can bedisadvantageous from an integration perspective, but may result in amore adiabatic transformation (expansion/compression) of the mode.Ultimately, this may reduce transition losses and radiation modes in thesecond region 205 of the waveguide and the optical fiber 208,respectively. Finally, it is of interest to note that angle θ₁ and theangle θ₂ are not necessarily equal. Illustratively, the angle θ₂ isgreater than angle θ₁.

The length of taper of lower portion 206 (shown in FIG. 2(b) as L₂) ison the order of approximately 100 μm to approximately 1,500 μm. Ofcourse, FIG. 2(b) is not drawn to scale as the width of the waveguide(shown as w_(g)) is hundreds of times smaller than the length L₂ of thetaper portion (e.g. 1-10 microns wide). The length of the taper of theupper portion 207 of the waveguide (shown at L₁) is on the order ofapproximately 100 μm on to approximately 1,500 μm. As described above,smaller taper angles will result in longer taper lengths (L₁) andconsequently may require more chip surface area, which can be lessdesirable in highly integrated structures. However, the length of thetaper (L₁) also dictates the efficiency of the mode shaping. To thisend, longer tapers may provide more efficient mode shaping because themode transformation is more adiabatic.

In the illustrative embodiment of FIGS. 2(a) and 2(b), the upper portion207 and the lower portion 206 of guiding layer 203 are substantiallysymmetric about an axis 213 that bisects the guiding layer 203. As such,the first angle θ₁ of the lower portion is the same on both sides of theaxis 213. Similarly, second angle θ₂ of upper portion is the same onboth sides of the axis 213. In the present embodiment in which the upperportion 207 and lower portion 206 are symmetric about axis 213, thelengths L₁ and L₂ are the same on both sides of the axis 213.

Finally, as described below, the tapering of the waveguide reduces thewidth (w_(g)) of the guiding layer 203, which enables substantiallyadiabatic expansion/compression of the horizontal portion of the mode.At the endface 209, the width is reduced to a width w_(g) as shown.Illustratively, this width w_(g) is in the range of approximately 0.5 μmto approximately 2.0 μm. While the embodiment shows that guiding layer203 terminates at this width rather abruptly. Of course, as in theillustrative embodiment of FIGS. 1(a) and 1(b), it is possible tocontinue the guiding layer 203 at the reduced width, w_(g), for a finitelength, which ultimately terminates at an endface.

Fabrication of the waveguide 200 may be effected by relatively standardsemiconductor fabrication process technology. Particularly advantageousis the fact that the guiding layer 203 may be fabricated of a singlelayer, illustratively a single layer of a single material. To fabricatethe device shown illustratively in FIG. 1, a suitable material isdeposited on the substrate 201. This material is illustrativelymonolithic, and is deposited in a single deposition step. A conventionalphotolithographic step is thereafter carried out, and a conventionaletch, such as a reactive ion etching (RIE) technique may be carried outto form the waveguide 203 and to define the lower portion 206. The upperportion 207 may be fabricated by a second conventionalphotolithography/etch sequence.

Alternatively, a monolithic material may be deposited on layer 202, andin the deposition step, the taper in the lower portion 206 of secondregion 205 may be formed. After the deposition step, the guiding layer203 may be partially etched to form the taper in the top portion 207.The top portion 207 can be etched by standard dry or wet etchtechniques, both isotropically and anisotropically. While theillustrative embodiment described thus far is drawn to the guiding layer203 being formed of a single layer, it is clear that this waveguide maybe formed of multiple layers of a single material as well. To this end,the guiding layer 203 may be comprised of a lower layer which includesthe lower portion 206 and an upper layer (not shown) which includes theupper portion 207. In the technique in which two sequential layers aredeposited, the top layer is thereafter etched by standard technique toform the taper in the top portion 207 of the second region 205 of theguiding layer 203.

For purposes of illustration, and not limitation, in the illustrativeembodiment, the lower cladding layer 202 is silicon dioxide (SiO₂)having an index of refraction on the order of approximately 1.46. Theguiding layer is illustratively silicon oxynitride (SiO_(x)N_(y)), andthe upper cladding layer (not shown) is also SiO₂. In this illustrativeexample of materials, in the first region 204, guiding layer 203 has athickness (shown at t₁ in FIG. 2(a)) on the order of approximately 2.0μm to approximately 4.0 μm. As can be seen in FIG. 2(a), the thicknessof guiding layer 203 reduces from t₁ to t₂. Moreover, as can be seen inFIG. 2(a), at section 210 guiding layer 203 has a thickness t₁, which isthe sum of the thickness t₃ of upper portion 207 and thickness t₂ lowerportion 206. At section 211, the thickness of guiding layer 203 isreduced to t₂, which is the thickness of lower portion 206.

While the taper (reduction of the width, w_(g)) of the upper portion 207and lower portion 206 results in the adiabatic expansion of thehorizontal portion of the confined mode, the reduction in the thicknessfrom t₁ to t₂ results in the adiabatic expansion of the vertical portionof the confined mode. As described above, the reduction of the thicknessof the guiding layer 203 results in a reduction in the effective indexof refraction (n_(eff)) for the vertical portion of the mode. As such,the mode is less confined vertically in the guiding layer 203, and isprogressively expanded as it traverses the waveguide 200 in the+z-direction. At endface 209, the mode is effectively matched to theguiding layer characteristics of optical fiber 208. The lower portion206 has an illustrative thickness (t₂) in the range of approximately 1.0μm to approximately 2.0 μm. Finally, the upper portion 207illustratively has a thickness (t₃) in the range of approximately 1.0 μmto approximately 2.0 μm. FIGS. 3(a) and 3(b) show the electric fielddistribution of the confined mode in the first portion 204 of waveguide200 along the x-axis at a point z₀ and along the y-axis at point z₀,respectively. Stated differently FIG. 3(a) shows the horizontal portionof the electric field of the confined mode in first region 204, whileFIG. 3(b) shows the vertical portion of the electric field of the mode.As can be appreciated, the mode energy is particularly confined in thefirst region 204 of the waveguide 200. Characteristically, this is anenergy distribution of a supported eigenmode of a planar waveguide (notshown), which is readily coupled to the first region 204 of waveguide200 having virtually the same physical characteristics as the planarwaveguide.

FIGS. 3(c) and 3(d) show the electric field of the confined mode in thesecond region 205 of the waveguide 200, particularly near point 212.More particularly, FIGS. 3(c) and 3(d) show the horizontal and verticalportions of the electric field distribution of the confined mode,respectively, in second region 205 of waveguide 200. As can be seen, thesupported mode in this portion of waveguide 200 is slightly expanded(less confined to the guiding layer 203) compared to the supported modein the first portion 204.

FIGS. 3(e) and 3(f) show the horizontal and vertical portion of theelectric field distribution, respectively, of the confined mode atapproximately endface 209 of the second region 205 of waveguide 200. Atthis point, the electric field distribution of the confined mode issignificantly greater in both the horizontal direction (FIG. 3(e)) andthe vertical direction (FIG. 3(f)). The adiabatic transformation of themode from the relatively confined mode of the first region 204 to therelatively expanded mode at endface 209 is relatively adiabatic, andresults in transition losses which are substantially negligible.

A review of FIGS. 3(a)-3(f) reveals the adiabatic expansion of theconfined mode traversing the guiding layer 203 in the +z-direction. Asreferenced above, the tapers of the lower portion 206 and the upperportion 207 result in a reduction in the width, wg, of guiding layer203. This results in a reduction in the effective index of refraction(n_(eff)) for the horizontal portion of the mode. As such, thehorizontal portion of the mode is less confined to the guiding layer203. Accordingly, the mode is expanded as it traverses the waveguide200. Additionally, the reduction in the thickness of the guiding layer203 from t₁ to t₂ results in a reduction in the effective index ofrefraction (n_(eff)) for the vertical portion of the mode. As such, themode is less confined in the guiding layer 203. The mode as representedin FIGS. 3(d) and 3(e) will be supported by an optical fiber.

EXAMPLE II

As described above, the upper portion and lower portion of the guidinglayer in Example I were substantially symmetric about an axis bisectingthe guiding layer. In the illustrative embodiments of Example II, theupper portion of the guiding layer may be asymmetric about an axisbisecting the guiding layer. The lower portion may be symmetric aboutthe axis bisecting the guiding layer. Alternatively, both the upperportion and the lower portion may be asymmetric about an axis bisectingthe guiding layer. The asymmetry of either the upper portion of theguiding layer alone or of the upper and lower portions of the guidinglayer about an axis which bisects the guiding layer may be beneficialfrom the perspective of manufacturing and fabrication.

In the illustrative embodiments described herein, the asymmetry of thetaper of either the upper portion or the upper portion and lower portionof the guiding layer offers more tolerance during fabrication. To thisend, mask positioning tolerances are greater when fabricating tapersthat are asymmetric. It is of interest to note that standard masking andetching steps described in connection with the illustrative embodimentsin Example I may be used in fabricating the waveguides of theillustrative embodiments of the present example. Moreover, as describedin connection to the illustrative embodiments of Example I, waveguidesaccording to the illustrative embodiment facilitate efficient opticalcoupling between two waveguides by adiabatically expanding/compressingan optical mode. Again, waveguides according to the exemplaryembodiments of Example II illustratively couple optical fibers of anoptical communication system to planar waveguides of an OIC.

Turning to FIG. 4(a), a top view of guiding layer 401 of a waveguide isshown. Again, a lower cladding layer (not shown) and an upper claddinglayer (not shown) may be disposed under and over the guiding layer 401,respectively, thereby forming a waveguide. The upper and lower claddinglayers are substantially the same as described in connection with theillustrative embodiments described fully above. A lower portion 402 ofguiding layer 401 has a lower portion first taper 403 and a lowerportion second taper 404. The lower portion first taper 403 is definedby an angle θ₃ and length 405. The length 405 of the lower portion firsttaper 403 is readily determined by dropping a perpendicular to theterminal point of the first taper 403. Lower portion second taper 404 isdefined by an angle θ₄, a length 406, again defined by dropping aperpendicular to the terminal point. An upper portion 407 of guidinglayer 401 is disposed on the lower portion 402. The upper portion 407has an upper portion first taper 408 which is defined by an angle θ₁ anda length 410, which may be found by dropping a perpendicular from theterminal point of upper portion first taper 408. Similarly, an upperportion second taper 409 of guiding layer 401 is defined by angle θ₂ anda length 411, which is determined by dropping a perpendicular from theterminal point of the taper to the edge of the guiding layer 401 asshown. The guiding layer 401 has an illustrative width w_(g), whichdecreases to a width w_(g) at endface 410. The section 411 of guidinglayer 401 has a constant width w_(g). Section 412 is illustrative, andthe endface having reduced width w_(g) may be located at the terminationof lower portion 402.

In the illustrative embodiment of FIG. 4(a), an axis 413 bisects theguiding layer 401. The upper portion 407 is asymmetric about the axis413. Contrastingly, the lower portion 402 is substantially symmetricabout the axis 413. In the illustrative embodiment of FIG. 4(a), theangles θ₁ and θ₃ are dissimilar, and the taper lengths 410 and 411 oftapers 408 and 409, respectively, are also dissimilar. However, in theillustrative embodiment of FIG. 4(a), the angles θ₃ and θ₄ aresubstantial identical The lengths 405 and 406 of lower portion first andsecond tapers 403 and 404, respectively, are substantially identical, aswell. Advantageously, the constraints on mask location tolerances informing the upper portion 407 of guiding layer 401 are lessened, whencompared to the embodiments described above where the upper portion issymmetric about an axis that bisects the guiding layer 407.

As can be readily appreciated, by varying angle θ₃ of upper portionfirst taper 403 and length 405 of lower portion first taper 403; byvarying angle θ₄ of lower portion second taper 404 and length 406 oflower portion second taper 404; by varying angle θ₁ of upper portionfirst taper 408 and length 410 of upper portion first taper 408; and byvarying angle θ₂ of upper portion second taper 409 and length 411 ofupper portion second taper 409, a variety of structures for guidinglayer 401 may be realized. The results may be that the upper portion isasymmetric about axis 413, while the lower portion 402 is symmetricabout axis 413. Alternatively, both upper portion 407 and lower portion402 of guiding layer 401 may be asymmetric about axis 400. Someillustrative structures are described below. Of course, these are merelyexemplary and are in no way limiting of the present invention.

Turning to FIG. 4(b), a top view of an illustrative embodiment of thepresent invention is shown. In the illustrative embodiment of FIG. 4(b),the lower portion 402 of the guiding layer 401 is substantiallysymmetric about axis 413. That is, angle θ₃ is substantially identicalto angle θ₄, and the length 405 is substantially the same as secondlength 406. However, angle θ₂ and length 411 are essentially zero. Assuch, there is no second taper of upper portion 407. Upper portion 407is substantially defined by θ₁, and length 410. This embodiment isparticularly advantageous in that a mask used to define the upperportion 407, need be only semi-self-aligning. That is it need onlyintersect the lower portion 402, since the taper of upper portion 407 isone-sided and terminates at a point at the edge of lower portion 402.This absence of a second taper results in a lower need for accuracy inmask alignment.

Turning to FIG. 4(c), another illustrative embodiment of the presentinvention is shown. Guiding layer 401 includes lower portion 402 andupper portion 407. In this illustrative embodiment, angles θ₁ and θ₄ areessentially zero. Upper portion 407 includes upper portion second taper409 having a taper length 411. Lower portion 402 has a first taper 403having a taper length 405.

According to this illustrative embodiment, both the upper portion 407and the lower portion 402 are asymmetric about axis 413 that bisects theguiding layer 401.

Turning to FIG. 4(d), another illustrative embodiment of the presentinvention is shown In this illustrative embodiment both the lowerportion 402 and the upper portion 407 of the guiding layer 401 areasymmetric about an axis 413 that bisects the guiding layer 401. Again,angles θ₁ and θ₂, in conjunction with lengths 410 and 411, may be usedto define the taper of upper portion 407. Similarly, the angle θ₃ andlength 405 may be used to define the taper of the lower portion 402 ofguiding layer 401.

As can be readily appreciated from a review of the illustrativeembodiments of Example II, the guiding layer may be of a variety ofstructures. The embodiments described are merely exemplary of thewaveguide of the present invention. As such, these exemplary embodimentsare intended to be illustrative and in no way limiting of the invention.

EXAMPLE III

In the present example, other illustrative embodiments of the presentinvention are described. These illustrative embodiments may incorporatethe principles of symmetry and asymmetry of the guiding layer asdescribed above. Moreover, many of the fabrication techniques describedin connection with the illustrative embodiments of Examples I and II maybe used.

FIG. 5 shows a perspective view according to another illustrativeembodiment of the present invention. A waveguide 500 includes a lowercladding layer 502. The lower cladding layer 502 way he disposed on asubstrate 501. A guiding layer 503 is disposed on lower cladding layer502. An upper cladding layer (not shown) may be disposed on the guidinglayer 503. In the embodiment shown in FIG. 5, the lower portion 507 ofthe guiding layer 503 is a diffused guiding layer. In the particularembodiment shown in FIG. 5, the lower portion 507 is illustratively aTi:LiNbO₃ waveguide. The top portion 506 of waveguide 503 is a materialhaving an index of refraction that is substantially the same as that ofthe lower portion 507 (the diffused waveguide). Advantageously, theembodiment shown in FIG. 5 is useful because diffused guiding layers areoften wider (along x axis) than they are deep (along y axis). The secondregion 505 of the top portion 506 is tapered in the manner similar tothat shown in previous embodiments, for example that of FIG. 1. The topportion 506 of the second region 505 is useful in providing bothvertical and horizontal mode transformation.

Turning to FIG. 6, another illustrative embodiment of the presentinvention is shown. In this illustrative embodiment waveguide 600 has asecond region 605 that illustratively includes three layers. Of course,this is merely illustrative, and more layers are possible. The substrate601 has a lower cladding layer 602 disposed thereon. The guiding layer603 has a first region 604 and a second region 605. The second region605 has a lower portion 606 and an intermediate portion 607 and a topportion 610. An upper cladding layer 611 (not shown) may be dispose overguiding layer 603. Again, a waveguide couples to the end face 608; andillustratively the waveguide is an optical fiber (not shown). In theillustrative embodiment shown in FIG. 6, the second region 605 issymmetric about an axis 609 which bisects the lower portion 606. Thefabrication sequence and materials are substantially the same in theembodiment shown in FIG. 6. Of course, a third photolithography/etchingstep would have to be carried out in the embodiment in which one layerof material is deposited to form the guiding layer 603. Of course,multiple depositions of the same material could be carried out in amanner consistent with that described in connection with FIG. 1.Thereafter, a sequence of photolithographic and etching steps would becarried out to realize the lower portion 606, intermediate portion 607and top portion 610 of the second region 605.

In the foregoing examples, waveguides have been described as being madewith tapers that vary in horizontal width, that is, width that changesin the direction of the plane of the substrate that the waveguide isfabricated on. This is an advantage of the invention, for whilewaveguides with vertical taper could also be fabricated as an embodimentof the present invention, these are much more difficult to manufacture.

The invention having been described in detail in connection through adiscussion of exemplary embodiments, it is clear that variousmodifications of the invention will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure. Suchmodifications and variations are included within the scope of theappended claims.

1. An optical device for transforming a waveguide mode from a first modeshape to a second mode shape, the device comprising: a single-modewaveguide comprising a guiding layer comprising a single material, theguiding layer configured to support a single optical mode at one end ofthe guiding layer and configured to support a single optical mode at anopposing end of the guiding layer, the guiding layer having a firstwidth at an end of the guiding layer and a relatively lesser secondwidth at an opposing end of the guiding layer for transforming ahorizontal portion of the waveguide mode, the guiding layer comprising alower guiding portion comprising the single material and having a firsttaper in width of the lower guiding portion and an upper guiding portioncomprising the single material and having a second taper in width of theupper guiding portion, the upper guiding portion disposed on the lowerguiding portion.
 2. An optical device as recited in claim 1, wherein theguiding layer comprises a rib waveguide cross-sectional shape at a firstend of the guiding layer and comprises a non-rib waveguidecross-sectional shape at an opposing second end of the guiding layer. 3.An optical device as recited in claim 2, wherein the guiding layer has afirst thickness at an end of the guiding layer and a relatively lessersecond thickness at an opposing end of the guiding layer fortransforming a vertical portion of the waveguide mode.
 4. An opticaldevice as recited in claim 2, wherein the guiding layer comprises anintermediate portion having a third taper in width of the intermediateportion, the intermediate portion disposed between the lower portion andthe upper portion.
 5. An optical device as recited in claim 1, whereinthe upper and lower portions comprise the same refractive index.
 6. Anoptical device as recited in claim 1, wherein the first taper is at afirst angle and the second taper is at a second angle different from thefirst angle, and wherein at least one of the first angle and secondangle is in the range of approximately 0° to approximately 0.5°.
 7. Anoptical device as recited in claim 1, wherein the guiding layer has afirst thickness at an end of the guiding layer and a relatively lessersecond thickness at an opposing end of the guiding layer fortransforming a vertical portion of the waveguide mode.
 8. An opticaldevice as recited in claim 1, wherein the guiding layer comprises anintermediate portion having a third taper, the intermediate portiondisposed between the lower portion and the upper portion.
 9. An opticaldevice as recited in claim 1, wherein the upper portion is asymmetricabout an axis which bisects the lower portion.
 10. An optical device asrecited in claim 7, wherein the first thickness is approximately 2.0micrometers to approximately 4.0 micrometers, and the second thicknessis approximately 1.0 micrometers to approximately 2.0 micrometers. 11.An optical device as recited in claim 1, comprising a stresscompensating layer on which the guiding layer is disposed.
 12. Anoptical device for transforming a waveguide mode from a first mode shapeto a second mode shape, the device comprising: a single-mode waveguidecomprising a guiding layer comprising a rib waveguide cross-sectionalshape at a first end of the guiding layer and comprising a non-ribwaveguide cross-sectional shape at an opposing second end of the guidinglayer, the guiding layer having a first width at an end of the guidinglayer and a relatively lesser second width at an opposing end of theguiding layer for transforming a horizontal portion of the waveguidemode, the guiding layer comprising a lower guiding portion having afirst taper in width of the lower guiding portion and an upper guidingportion having a second taper in width of the upper guiding portion, theupper guiding portion disposed on the lower guiding portion.
 13. Anoptical device as recited in claim 12, wherein the upper and lowerportions comprise the same refractive index.
 14. An optical device asrecited in claim 12, wherein the first taper is at a first angle and thesecond taper is at a second angle different from the first angle, andwherein at least one of the first angle and second angle is in the rangeof approximately 0° to approximately 0.5°.
 15. An optical device asrecited in claim 12, wherein the guiding layer has a first thickness atan end of the guiding layer and a relatively lesser second thickness atan opposing end of the guiding layer for transforming a vertical portionof the waveguide mode.
 16. An optical device as recited in claim 12,wherein the guiding layer comprises an intermediate portion having athird taper, the intermediate portion disposed between the lower portionand the upper portion.
 17. An optical device as recited in claim 12,wherein the upper portion is asymmetric about an axis which bisects thelower portion.
 18. An optical device as recited in claim 15, wherein thefirst thickness is approximately 2.0 micrometers to approximately 4.0micrometers, and the second thickness is approximately 1.0 micrometersto approximately 2.0 micrometers.
 19. An optical device as recited inclaim 12, comprising a stress compensating layer on which the guidinglayer is disposed.
 20. An optical device as recited in claim 12, whereinthe guiding layer comprises a single material.